2.5mm jacks exist but iOS devices have never used them. I think some feature phones did that back in the day. Personally I've only encountered them on a pair o

2.5mm jacks exist but iOS devices have never used them. I think some feature phones did that back in the day. Personally I've only encountered them on a pair of headphones where they're a step up from a soldered on cable, but more annoying than a standard jack would be. It's also recessed really far in with a very narrow twist-lock connector, so isn't compatible with any cables except the special one it comes with.